1. Field of the Invention
The present invention relates to a method of achieving ultra-low NOx without sacrificing thermal efficiency for hydrogen fueled gas turbines. In particular, the present invention comprises a method for minimizing the need to extract compressor discharge air to limit mass flow through the power turbine, as is typically required in low NOx systems to maintain turbine rotor stress levels within acceptable limits. Systems of the present invention allow increased turbine inlet temperature, further improving system efficiency.
2. Brief Description of the Related Art
With energy usage directly related to economic growth, there has been a steady increase in the need for increased energy supplies. In the U.S., coal is abundant and low in cost. Unfortunately, conventional coal-fired steam plants, which are a major source of electrical power, are inefficient and pollute the air. Thus, there is a pressing need for cleaner, more efficient coal-fired power plants.
Accordingly, Integrated Gasification Combined-Cycle (IGCC) systems have been developed which can achieve significantly improved efficiencies in comparison to conventional steam plants. In such a system, syngas (a mixture comprising hydrogen and carbon monoxide) is produced by partial oxidation of coal or other carbonaceous fuel. This allows cleanup of sulfur and other impurities before combustion. Further, the carbon monoxide can be reacted with steam using the water gas shift reaction to form carbon dioxide and hydrogen. Carbon dioxide may then be recovered using conventional technologies known in the art. This allows pre-combustion recovery of carbon dioxide for sequestration and produces a high hydrogen content fuel. Gas turbines may also be powered by hydrogen produced in heavy oil recovery for upgrading the oil and in various chemical processes as a by-product.
Regardless of the source or purity, hydrogen poses an emission problem on combustion. Even without removal of carbon from syngas, the hydrogen content of the gas is typically too high to allow use of conventional dry low NOx premixed combustion for NOx control. Hydrogen ignition delay is short and the flame speed is high. Therefore, diffusion flame combustion is used with diluent added to the gas to minimize NOx by lowering the adiabatic flame temperature. Even so, exhaust gas cleanup may still be required. Thus, such systems cannot achieve present standards for NOx emissions without removal of NOx from the exhaust gas and the consequent efficiency loss. Improved combustion systems are needed.
There are further efficiency loss issues. If nitrogen is used to dilute the fuel gas, there is an energy penalty for nitrogen compression to the pressure needed for mixing with the fuel gas. Typically, avoiding excessive loads on the turbine rotor requires operation at a reduced turbine temperature and/or bleed of compressed air from the turbine compressor.
It is therefore an object of the present to provide a method for achieving ultra low NOx emissions in combustion of high purity hydrogen and syngas in gas turbines. It also an object of the present invention to provide a method for improving the thermal efficiency of power systems wherein the fuel comprises hydrogen.